Nov 22, 2011

Abstract of CCNA study guide-23 - IP Routing 2

Dynamic Routing
Two types of routing protocols are used in internetworks: interior gateway protocols (IGPs) and exterior gateway protocols (EGPs).
IGPs are used to exchange routing information with routers in the same autonomous system (AS).
EGPs are used to communicate between ASes.

Routing Protocol Basics
There are some important things you should know about routing protocols before getting
deeper into RIP. Specifically, you need to understand administrative distances, the three different
kinds of routing protocols, and routing loops.
Administrative Distances
-        The administrative distance (AD):  is used to rate the trustworthiness of routing information received on a router from a neighbor router.
-        An administrative distance is an integer from 0 to 255, where 0 is the most trusted and 255 means no traffic will be passed via this route.
-        If a router receives two updates listing the same remote network, the router check First the AD. If one of the advertised routes has a lower AD than the other, then the route with the lowest AD will be placed in the routing table.
-        If both advertised routes to the same network have the same AD, then routing protocol metrics (such as hop count or bandwidth of the lines) will be used to find the best path to the remote network.
-        The advertised route with the lowest metric will be placed in the routing table. But if both advertised routes have the same AD as well as the same metrics, then the routing protocol will load-balance to the remote network (which means that it sends packets
-        down each link).

TABLE 6 . 2 Default Administrative Distances
Route Source
Default AD
Connected interface
Static route
External EIGRP

Routing Protocols
There are three classes of routing protocols:

Distance vector : that find the best path to a remote network by judging distance.
Each time a packet goes through a router, that’s called a hop. The route with the least number of hops to the network is determined to be the best route.
Both RIP and IGRP are distance-vector routing protocols.

Link state : each router create three separate tables.
Neighbors: keeps track of directly attached neighbors
Topology : determines the topology of the entire internetwork
Routing table :
OSPF is an IP routing protocol that is completely link state.

Hybrid Hybrid protocols use aspects of both distance vector and link state—for example, EIGRP.

Distance-Vector Routing Protocols
The distance-vector routing algorithm passes complete routing table contents to neighboring
routers, which then combine the received routing table entries with their own routing tables to
complete the router’s routing table.

Routing Loops
-        the slow convergence of distance-vector routing protocols can result in inconsistent routing tables and routing loops.
-        Routing loops can occur because every router isn’t updated simultaneously.

Example: let’s say that the interface to Network 5 in Figure 6.15 fails. All routers know about Network 5 from RouterE. RouterA, in its tables, has a path to Network 5 through

When Network 5 fails, RouterE tells RouterC. But routers A, B, and D don’t know about Network 5 yet, so they
keep sending out update information. RouterC will eventually send out its update and cause B to stop routing to Network 5, but routers A and D are still not updated. To them, it appears that Network 5 is still available through RouterB with a metric of 3.
The problem occurs when RouterA sends out its regular route table  which includes the ability to reach Network 5, and now routers B and D receive that Network 5 can be reached from RouterA, so routers B and D then send out the information that Network 5 is available. Any packet destined for Network 5 will go to RouterA, to RouterB, and then back to RouterA. This is a routing loop—how do you stop it?

Maximum Hop Count
One way of solving this problem is to define a maximum hop count. RIP permits a hop count of up to 15, so anything that requires 16 hops is deemed unreachable.
Split Horizon
enforcing the rule that routing information cannot be sent back in the direction from which it was received.
Route Poisoning
For example, when Network 5 goes down, RouterE initiates route poisoning by advertising Network 5 as 16, or unreachable
This poisoning of the route to Network 5 keeps RouterC from being susceptible to incorrect updates about the route to Network 5. When RouterC receives a route poisoning from RouterE, it sends an update, called a poison reverse, back to RouterE. This ensures that all routes on the segment have received the poisoned route information.
A holddown prevents regular update messages from reinstating a route that is going up and down (called flapping).

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